1. On behalf of the BABAR Collaboration
Search for DARK Force at BABAR
APS April Meeting 2018, Columbus, Ohio
Introduction to the DARK force
Search for muonic DARK force
Search for invisible DARK photon decay
Summary/Conclusions
Richard Kass
On behalf of the BABAR Collaboration
4/15/2018
Richard Kass

2. Search Motivation-I Ample evidence for DARK MATTER (DM)
gravitational mass≠luminous mass
Galactic Rotation Curves
Fritz Zwicky (1933)
Coma galaxy cluster
Large Scale Structure
CMB
Bullet Cluster
Generated by Amanda Yoho, Planck CMB simulator
X-ray: NASA / CXC / CfA / M.Markevitch et al.; Optical: NASA / STScI; Magellan
/ U.Arizona / D.Clowe et al.; Lensing Map: NASA / STScI; ESO WFI; Magellan
/ U.Arizona / D.Clowe et al.
Sloan Digital Sky Survey 1.25 Declination Slice 2013 Data”
by M. Blanton and the Sloan Digital Sky Survey
4/15/2018
Richard Kass

3. Search Motivation-II Lots of indirect evidence of DM from Astronomy
We’d like some direct evidence…..
Non-accelerator DM experiments: detect DM hitting a target
LUX, PandaX, CDMS, PICO, CRESST, etc
Accelerator experiments: produce & detect DM
CMS, ATLAS
In spite of intense effort no direct detection of DM
Maybe we should expand our search horizon….
4/15/2018
Richard Kass

4. Can perform DS searches with BaBar
The Dark Sector (DS)
Move beyond the WIMP paradigm, go to the DARK sector
What is the DARK sector?
The DARK sector includes particles that couple weakly to the SM
Theory motivated by BSMs, string theory, extra U(1) group
Dark force mediated by DARK U(1) photon(s), A’
DARK matter could be part of the DARK sector
No reason to believe that there is only one DM particle
DARK sector could have a rich spectroscopy with DM just a piece of it
Light DM models (sub-GeV) require new low mass mediators to explain the observed relic density. Can explain with DARK sector.
DARK photon couples to SM fermion with strength: α’=ε2αEM
Can perform DS searches with BaBar
4/15/2018
Richard Kass

5. Search for muonic DARK force @ BaBar
> A DARK force that couples to 2nd and 3rd generation SM particles
Z’/g-2: X. G. He et al., Phys. Rev. D 43, 22 (1991). X. G. He et al., Phys. Rev. D 44, 2118 (1991)
New gauge boson, Z’, could be low mass (Shuve, Yavin, PRD 89, (2014))
> Could explain anomalies such as g-2, dark matter as sterile v’s, proton radius discrepancy.
Search for Z’ via bremsstrahlung
4 muon final state
CCFR: νμN®μ+μ-X
Existing limits in region accessible to BaBar GeV
Trident: W. Altmannshofer etal., PRL. 113, (2014).
A. Kamada and H. B. Yu, PRD 92, (2015).
4/15/2018
Richard Kass

6. Search for muonic DARK force @ BaBar
Analysis Strategy
> Use 514 fb-1 of data collected at Y(2S), Y(3S), Y(4S)
> Select events with 4 charged tracks little/no extra energy (< 200MeV)
> At least 2 same sign tracks passing muon criteria
> |m4μ-ECM| ≤ 500 MeV
> Veto Y(2S, 3S)-> ππ Y(1S), Y(1S)->μμ candidates
> Kinematic fit with a beam energy constraint
> Selection criteria optimized on 5% of the data, rest of analysis is blind
Signal efficiency is function of mR, varies from 35% to 50%
mR=(m2μ μ-4m2μ )½
blue line is a fit ρ
ρ reflection
> Signal region dominated by e+e-->4μ background
Discrepancy due to no ISR in MC, PID efficiencies, track reconstruction.
Discrepancy not important since looking for a peak over background
4/15/2018
Richard Kass

7. Results Search for muonic DARK force @ BaBar
Calculate 90% CL Bayesian Upper limits for σ(e+e-->μ+μ-Z’, Z’->μ+μ-)
Combine likelihoods from Y(2S), Y(3S), Y(4S) samples
Includes systematic uncertainties:
signal + bkg modeling, luminosity, etc
Stringent upper limits on coupling constant ~ 7x10-4 near μ+μ- threshold
Assume identical coupling strengths for muons, taus, corresponding neutrinos
Can be interpreted as constraint on NP vector fields coupling to muons
Rules out most of “g-2” favored region
BaBar
BaBar
Published in Phys. Rev. D. 94,
4/15/2018
Richard Kass

8. Search for invisible decay of a DARK photon (A’)
DARK photon will decay to invisible (undetectable) states if lighter DS states exist
At BaBar we search for e+e--> γA’, A’-> invisible by tagging the photon
BaBar collected ~53fb-1 of data with dedicated single photon triggers
mostly at Y(2S) and Y(3S) energies
Signature is an event with missing energy and missing momentum
BaBar is almost hermetic
Select single-photon events, look for bump in missing mass, MX or Eγ
Main backgrounds are from QED sources:
e+e-->γγ, e+e-->e+e-γ where one or more of particles is outside acceptance
4/15/2018
Richard Kass

9. Analysis details: invisible decay of a DARK photon
Divide data into two subsets by photon triggers:
Low mass: -4 <MX2 <36 GeV2
High mass: 36 <MX2 <69 GeV2
Detected photon criteria: energy, angles in lab and CM
Zero charged tracks with CM momentum above:
1 GeV for low mass
0.1 GeV for high mass
Minimum extra energy in calorimeter
Missing 4-vector
Direction of missing vector
Fiducial cuts
Angular distance between missing 4-vector and EMC crystal edge
Hadron calorimeter + muon veto
Train BDT to discriminate signal/background in low/high mass regions
Low mass: residual bkgd from e+e-->γγ limits sensitivity
High mass: smooth background
BaBar
4/15/2018
Richard Kass

10. Search for invisible decay of a DARK photon, A’ More Analysis Details
Need to further reduce peaking contribution from e+e-->γγ near mX ~0
Define several signal regions in 2D plot, photon angle vs BDT output
Tight Cut: reduce peaking contribution from e+e-->γγ near mX ~0
Loose Cut: optimize for observation (smooth bkdg)
Background: pure background region
Use 4 non-overlapping regions for each data set
Low-mass+tight cut
Low-mass+loose-cut and Not tight cut
High mass+loose cut
Background
Total of 9 low-mass and 4 high mass data sets
Signal efficiency varies as a function of mA’ lowest 0.2%, highest 3.8%
Signal Search
Extract number of signal events by a simultaneous ML fit to the
independent regions for each data set (Y(2S),Y(3S), Y(4S))
166 mass hypotheses
For each fit: bkg shape is fixed, signal yield, peaking & contin. bkgs floated
4/15/2018
Richard Kass

11. Results: No significant signal
Search for invisible decay of a DARK photon, A’
Results: No significant signal
Large improvement over previous searches
Rules out entire region preferred by (g-2)μ anomaly
90% CL on ε2 vs MA’
90% CL on ε vs MA’
BaBar
PRL 119, (2017)
4/15/2018
Richard Kass

12. Summary/Conclusions
Lack of direct evidence of DM  Consider DARK force
Older BaBar searches looking for DARK particles:
Search for dark photon in e+e-→γ A‘, A'→ e+e-, μ+μ- PRL 113 (2014)
Search for dark Higgs boson in e+e- → h'A' h' → AA‘ PRL 108 (2012)
Search for dark boson in e+e- → γA'→W'W‘ arXiv:
Today reported on latest results from two searches:
Muonic DARK force Phys. Rev. D. 94,
Invisible decay of a DARK photon (A’) PRL 119, (2017)
No evidence for signal(s), report 90 % CL ULs in mass range < 10 GeV
Ongoing BaBar efforts in this area:
Search for DARK scalar (φ)
Search for self-interacting DM: darkonuim  multi muon final states
4/15/2018
Richard Kass

13. Extra Slides
4/15/2018
Richard Kass

14. BaBar Data Set Took data 1999-2008 at PEP-II asymmetric e+e- collider:
~1.3x109 e+e- →cc, ~0.9x109 e+e- →τ+τ-, ~0.5x109 e+e- →BB, ~0.2x109 e+e- →bb
CUSB
4/15/2018
Richard Kass

15. BaBar Detector Took data 1999-2008 at PEP-II asymmetric e+e- collider
~1.3x109 e+e- →cc, ~0.9x109 e+e- →τ+τ-,
~0.5x109 e+e- →BB, ~0.2x109 e+e- →bb
1.5 T Solenoid
Electromagnetic Calorimeter (EMC)
Detector of Internally Recflected Cherenkov Light (DIRC)
Instrumented Flux Return (IFR)
Silicon Vertex Tracker (SVT)
Drift Chamber (DCH)
e- (9 GeV)
e+ (3.1 GeV)
SVT, DCH: charged particle tracking: vertex & mom. resolution, K0s/Λ
EMC: electromagnetic calorimeter: g/e/π0/η
DIRC, IFR, DCH: charged particle ID: π/μ/K/p
Efficient trigger for DS searches
NIM A479, 1 (2002)
NIM A729, 615 (2013)
4/15/2018
Richard Kass

16. Search for muonic DARK force @ BaBar
Analysis Strategy
> Use 514 fb-1 of data collected at Y(2S), Y(3S), Y(4S)
> Select events with 4 charged tracks little/no extra energy (< 200MeV)
> At least 2 same sign tracks passing muon criteria
> |m4μ-ECM| ≤ 500 MeV
> Veto candidates with |m2μ-mY(1S)| ≤ 10 MeV to reject events from Y(2S),
Y(3S) data sets, i.e. Y(2S, 3S)-> ππ Y(1S), Y(1S)->μμ
> Kinematic fit with a beam energy constraint
> Selection criteria optimized on 5% of the data, rest of analysis is blind
Signal efficiency is function of mR, varies from 35% to 50%
Y(4S)
Signal region
mR=(m2μ μ-4m2μ )½
blue line is a fit ρ
ρ reflection
> Signal region dominated by e+e-->4μ background
Discrepancy due to no ISR in MC, PID efficiencies, track reconstruction.
Discrepancy not important since looking for a peak over background
4/15/2018
Richard Kass

17. Search for muonic DARK force @ BaBar
Separately calculate signal yields as function mZ’ for Y(2S, 3S, 4S) data sets
Perform unbinned ML using reduced mass, mR=(m2μ μ-4m2μ )½
2219 fits in range 0 <mR< 10 GeV,
mass window ~50X MZ’ resolution for mR > 0.2 GeV
mass range GeV for mR < 0.2 GeV
Signal shape from MC
Exclude region around J/ψ
σ(e+e-->μ+μ-Z’, Z’->μ+μ-)=Nsig /(Lεsig)
εsig from MC corrected with data
Z’ simulated with MadGraph + Pythia for several masses
interpolate between masses
Calculate significance SS=sgn(Nsig)(2lnL/L0)½
L=signal+background likelihood, L0=background only likelihood
J/ψ->μ+μ-
4/15/2018
Richard Kass

18. Search for invisible decay of a DARK photon, A’ More Analysis Details
Need to further reduce peaking contribution from e+e-->γγ near mX ~0
Define several signal regions in 2D plot, photon angle vs BDT output
Tight Cut: reduce peaking contribution
from e+e-->γγ near mX ~0
Loose Cut: optimize for observation (smooth bkdg)
Background: pure background region
Use 4 non-overlapping regions for each data set
Low-mass+tight cut
Low-mass+loose-cut and Not tight cut
High mass+loose cut
Background
Total of 9 low-mass and 4 high mass data sets
Signal efficiency varies as a function of mA’
lowest 0.2%, highest 3.8%
BDT
photon angle
4/15/2018
Richard Kass

19. Search for invisible decay of a DARK photon, A’
Look at data as a function of M2X
Blue curve: Bkdg only fit
Low mass region
Y(3S) Loose-Not Tight
Y(3S) Tight Cut
Irreducible peaking bkdg
from e+e-->γγ
M2X
M2X
High mass region
Y(3S) Loose
Y(2S) Loose
M2X
M2X
4/15/2018
Richard Kass

20. Search for invisible decay of a DARK photon, A’
Signal Search
Extract number of signal events by a simultaneous ML fit to the
independent regions for each data set (Y(2S),Y(3S), Y(4S))
166 mass hypotheses
For each fit background shape is fixed, signal yield, peaking
and continuum bkgs are floated
Local signal significance
Most significant fit, mA’=6.22 GeV
M2X
mA’
Local significance: 3.1σ
Global significance: 2.6σ
Global p-value ~ 1%
4/15/2018
Richard Kass

21. Previous BaBar DARK force results
Search for dark photon in
e+e-→γ A‘, A'→ e+e-, μ+μ-
Search for dark Higgs boson in
e+e- → h'A' h' → A' A'
PRL 113 (2014)
PRL 108 (2012)
Search for dark boson in
e+e- → γA' → W'W'
arXiv:
4/15/2018
Richard Kass